This article explains what a DIFF file is used for and how to open it on your computer.
Diff files in VS Code
A difference file records all the ways in which two text files differ. They are sometimes called patch files and may use the file extension PATCH.
This file type is typically used by software developers who are updating multiple versions of the same source code. Because it explains how the two versions differ, the program using the file can understand how to update the other files to reflect the new changes. Performing this type of change to one or more files is called patching the files.
Some patches can be applied to files even if both versions have changed. These are called context diffs, unified diffs, or unidiffs. Patches in this context are related to, but not the same as, software patches.